Methane production and emission from peat: the influence of anions (sulphate, nitrate) from acid rain

The influence of sulphate concentrations on the production and emission of methane in two contrasting peat sites was determined. Seasonal changes in sulphate concentrations appeared to influence the amount of organic carbon oxidised to carbon dioxide by sulphate reduction at both peat sites. For the majority of the year at both sites the amount of carbon mineralised through sulphate reduction exceeded that being transformed to methane by methanogenic bacteria, except when sulphate reduction became sulphate limited. In order to sustain the high sulphate reduction rates measured in the peat sulphide formed from dissimilatory sulphate reduction must be reoxidised rapidly to sulphate within the peat. Laboratory experiments showed that addition of 500 μM sulphate and 100 μM nitrate to peat samples significantly inhibited methanogenesis. Sulphate appeared to be the more important inhibitor of methanogenesis since inhibition of methane formation occurred with additions of sulphate reflecting in situ concentrations. Supplements of either acetate and/or hydrogen in combination with molybdate to peat samples revealed that methanogenesis was hydrogen limited and that the majority of active methanogens were hydrogen-utilising methanogens. Methanogenesis in peat samples appeared to be dependant on sulphate reducing bacteria for provision of substrates. Great Dun Fell, receiving the largest sulphate loading, had the lower rates of microbial activity (methane formation and sulphate reduction rates) than Ellergower, which received less than half the annual sulphate deposition of Great Dun Fell. This implied that some other factor—possibly organic matter lability, was limiting microbial rates of methane formation and sulphate reduction at Great Dun Fell.